Bcl-2 gene family and related proteins in mammary gland involution and breast cancer

The Bcl-2 gene family regulates tissue development and tissue homeostasis through the interplay of survival and death factors. Family members are characterized as either pro-apoptotic or anti-apoptotic, depending on cellular context. In addition to its anti-apoptotic effect, Bcl-2 also inhibits progression through the cell cycle. Functional interactions between family members as well as binding to other cellular proteins modulate their activities. Mammary gland tissue, similar to many other tissues, expresses a number of different Bcl-2 relatives including bcl-x, bax, bak, bad, bcl-w, bfl-1, bcl-2 as well as the bcl-2 binding protein Bag-1. Bcl-2 is expressed in the nonpregnant mammary gland and early pregnancy. In contrast, expression of bcl-x and bax continues through late pregnancy, is down-regulated during lactation, and upregulated with the start of involution. Bak, bad, bcl-w, and bfl-1 are also up-regulated during involution. The specific roles of individual gene products are investigated using dominant gain of function and loss of function mice. Finally, different Bcl-2 family members are commonly over- or under-expressed in human breast cancers. Bcl-2 expression in human breast cancers has been associated with a good prognosis, while decreased Bax expression has been linked to poor clinical outcome. Understanding the role Bcl-2 family members play in regulating mammary epithelial cell survival is salient to both normal mammary gland physiology and the development of new therapeutic approaches to breast cancer.